Ethyl Lead was put into gasoline to raise the octane.
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octane. It may not be what you think it is.
The Ethyl Lead after combustion settled out in the sump, as sludge. Lead deposited on valves and around the combustion chamber. Ethyl lead was an inexpensive solution to a problem It was likely the oil companies who started the BS about the lead being a valve seat saver. I have seen many engines with burned valves and seats. A piece of hardened lead deposit has held a valve open a few thousands of and inch. The flash heat of the combustion acted like a cutting torch and often cut a notch in the valve and the seat.
Leaded fuel was in use around the time of hydraulic valve lifers being installed in engines. Maybe, just maybe, some of the "country" mechanics set the valves incorrectly and there was a valve failure.
These old engines will always have a sludging situation, but, it will never be the same as it was in the bad-ol-daze.
Never discount the simple bypass oilfilter system. Eventually, all of the oil will have passed through the filter.
Back in the day, it was often truthfully said, "There are no old mechanics." It was the lead that was killing them. You will know you have lead poisoning when your pecker does not respond to stimulus.
https://www.medicalnewstoday.com/articles/306601.php